3 Climate and Environmental Physics, Physics Institute, University of - - PowerPoint PPT Presentation

Andrew Robertson1, Kenneth Carroll2, Chris Kubicki2, Roland Purtschert3

1USGS New Mexico Water Science Center, Albuquerque, New Mexico, USA 2New Mexico State University, Las Cruces, New Mexico, USA 3Climate and Environmental Physics, Physics Institute, University of Bern, Switzerland

Mesilla Basin/Conejos-Médanos Study Area

• Study area lies within the Rio Grande Rift Basin.
• General groundwater flow direction is towards the

river and from north to south.

• The Mesilla Basin is about 1,100 square miles (sq mi)

and includes as much as 200 sq mi in Chihuahua (Hawley and Kennedy, 2004)

• Santa Fe aquifer is up to 3,000 feet thick and consists
• f late Cenozoic alluvial deposits. The Sante Fe is
• verlain by recent river deposits.
• The conjunctive use of water in the Mesilla Basin

takes place under a myriad of legal constraints including the Rio Grande Compact, an international treaty, and a federal water project.

Generalized groundwater flow direction

Motivation for work in Mesilla Basin/Conejos-Médanos

• Declining water levels, deteriorating

water quality, and increasing use of water resources by municipal, industrial, and agricultural water users

• n both sides of the international

border have raised concerns about the long-term availability of the resource in the region (Alley, 2013).

• In 2014, initial NAWQA results suggest

that geothermal waters and brines are sources of natural contaminants to public supply wells in the Rio Grande aquifer system.

Dissolved-solids concentration at selected sites in the Rio Grande study area from 1934 to 1999 (Wilcox, 1968; Williams, 2001); from Moyer and

• thers, 2013

Mesilla Basin Reach

Geochemical and isotopic investigation of deep groundwater and salinity in the Mesilla Basin, New Mexico

Conventional groundwater sampling

• Sampled 17 nested wells in the Mesilla Valley (and 7 single-

completion wells in adjacent basins)

• Constituents:
• major ions,
• trace elements,
• stable isotopes of water (δ18O, δD),
• strontium (87Sr/86Sr),
• boron (δ11B) isotopes,
• uranium isotopes (234U/238U),
• carbon isotopes (δ13C and 14C) of dissolved inorganic

carbon,

• noble gas concentrations, and
• helium isotope (3He/4He) ratios.

Area of upward vertical gradients

Geochemical and isotopic investigation of deep groundwater and salinity in the Mesilla Basin, New Mexico

Conventional groundwater sampling – Residence Time

*pmC – Percent modern carbon relative to the NBS I oxalic acid standard, corrected for decay since 1950, but not normalized for 13C fractionation.

Helium accumulation

Modern Mixed

Geochemical and isotopic investigation of deep groundwater and salinity in the Mesilla Basin, New Mexico

Conventional groundwater sampling – Evaporite Dissolution

Saline, brackish & geothermal water (Witcher and

• thers, 2004)

200 400 600 800 1,000 1,200 1,400 1,600 1,800 2,000

• 500

1,000 1,500 2,000 2,500 3,000 Well depth (feet below land surface) Cl/Br

Fresh Brackish Saline

Geochemical and isotopic investigation of deep groundwater and salinity in the Mesilla Basin, New Mexico

Conventional groundwater sampling - Geothermal

Geochemical and isotopic investigation of deep groundwater in the Mesilla Basin, New Mexico

Ultra-trace groundwater sampling

• Sampled 8 wells within the Mesilla Valley for
• Krypton -81 (81Kr); half-life = 229,000 y
• Krypton -85 (85Kr); half-life = 10.756 y
• Argon -39 (39Ar); half-life = 269 y
• Conservative behavior, advantageous decay rates

Geochemical and isotopic investigation of deep groundwater in the Mesilla Basin, New Mexico

Ultra-trace groundwater sampling by degassing

• Current extraction efficiency is approximately 50%.
• ~1600 gallons to extract 40L of dissolved gases.
• Head loss of ~1.5 gpm primarily due to 250 mm filters
• General procedure

1. Leak test (~20 psi) with nitrogen, 2. Evacuate system with vacuum, 3. Purge system with dissolved groundwater gas, and 4. Collect sample.

• 85Kr and 39Ar is being analyzed by Low Level Counting (LLC) at

University of Bern.

• 81Kr and 85Kr is being analyzed at Argonne National Laboratory

with Atom Trap Trace Analysis (ATTA).

Geochemical and isotopic investigation of deep groundwater in the Mesilla Basin, New Mexico

Preliminary results

• Tritium results agree with 85Kr results

indicating no atmospheric contamination.

• The 81Kr analysis show no decay in 6

samples analyzed.

• The 39Ar piston flow ages were far

younger than the radiocarbon ages in the samples collected from wells in the southern basin.

• High Helium/Neon (He/Ne) indicate

mantle and radiogenic sourcing of He and possibly terragenic production of

39Ar.

*R/Ra is the ratio of 3He/4He in the sample to the ratio in air

Geochemical and isotopic investigation of deep groundwater in the Mesilla Basin, New Mexico

Summary

• Need for continued analysis of groundwater flow patterns, and salinity sources and

contributions

• Complex groundwater flow system without clear depth-dependent relations and multiple

sources of salinity

• Benefits of multi-tracer approach
• Distribution and source of dissolved solids
• Determine groundwater source and mixing
• Benefits of ultra-trace sampling approach
• Constrain age estimates
• Additional conservative tracer for confirming mixing models

Geochemical & isotopic investigation of deep groundwater and salinity in the Mesilla Basin/Conejos-Médanos, NM, TX, Chih.

Questions?